1. Field of Invention
The invention relates to an image forming apparatus including an intermediate medium, wherein an image is first formed on the intermediate medium by ink jet heads using phase-changeable ink, and then transferred onto a recording medium, more particularly, release oil is applied on the intermediate medium before the phase-changeable ink is ejected.
2. Description of Related Art
An ink jet printer, shown in FIG. 7, is known as the image forming apparatus described above, for example. FIG. 7 shows a schematic diagram of the main structure of the ink jet printer 80.
The ink jet printer 80 forms images using hot melt inks of yellow, magenta, cyan, and black. The ink jet printer 80 includes a drive roller 11, a following roller 12, an intermediate transfer belt 13, and a pair of sheet feed rollers 15.
The drive roller 11 is driven by a motor (not shown). The following roller 12 is provided opposite to and separate from the drive roller 11. The intermediate transfer belt 13 is wound around and spans between the drive roller 11 and the following roller 12. The sheet feed rollers 15 are disposed below the intermediate transfer belt 13 to feed a sheet P.
The ink jet head unit 20 is provided above the intermediate transfer belt 13 and includes an yellow head 21, a magenta head 22, a cyan head 23, and a black head 24, which eject yellow, magenta, cyan, and black colored inks respectively. The yellow head 21, the magenta head 22, the cyan head 23, and the black head 24 are provided above the intermediate transfer belt 13 such that their nozzle surfaces face to the intermediate transfer belt 13, and have nozzles which are oriented in the direction of the width of the intermediate transfer belt 13, that is they are line heads. Hot melt ink is ejected from each head onto the intermediate transfer belt 13 so that the color image is formed on the intermediate transfer belt 13.
A transfer roller 16 is disposed under the intermediate transfer belt 13 and, above the transfer roller 16, a heat roller 17 is provided opposite to the transfer roller 16 with the intermediate transfer belt 13 passing therebetween. The transfer roller 16 is movable in a direction indicated by an arrow F3 as the transfer roller 16 has a movement mechanism (not shown). On the left side of the transfer roller 16 is a separation pawl 18 provided for peeling the sheet S from the intermediate transfer belt 13. The separation pawl 18 is movable in a direction indicated by an arrow F4 as the separation pawl has a movement mechanism (not shown). A belt marker sensor 14 is provided on the left side of the ink jet head unit 20 in order to detect that the intermediate transfer belt 13 has reached a start position where an image is formed.
A silicon oil applying unit 90 is provided substantially below the following roller 12 and includes an oil tank 92 storing silicon oil 91 in which a felt 93 is soaked. The felt 13 is provided such that one end extends from an open mouth provided in the upper area of the oil tank 92 so as to contact the intermediate transfer belt 13. The upper end of the felt 93, contacting the intermediate transfer belt 13, has a width corresponding to the width of the intermediate transfer belt 13. A bracket 94 for a bolt is provided in the upper area of the oil tank 92 and a plate 95 is attached to the bracket 94 so as to be able to pivot. The upper end of the felt 93 is attached to a fore end of the plate 95 and an upper end of a coil spring 96 is attached to a rear end of the plate 95. A lower end of the coil spring 96 is connected to a plunger 98 of a solenoid 97.
When the ink jet printer 80 described above is turned on, the intermediate transfer belt 13 is transported by rotation of the drive roller 11. Then the solenoid 97 of the silicon oil applying unit 90 is driven so that the plunger 98 is pulled downward. As a result, the plate 95 pivots in the counterclockwise direction, as shown in FIG. 7, so that the end of the felt 93 is pushed against the intermediate transfer belt 13. In this way, silicon oil that soaks into the felt 93 is applied to the surface of the intermediate transfer belt 13. Because the width of the upper end of the felt 93 corresponds to the width of the intermediate transfer belt 13 as described above, silicon oil 91 is applied to the entire area of the intermediate transfer belt 13.
When the belt marker sensor 14 detects that the intermediate transfer belt 13 has reached a start position where an image is formed, the ink jet head unit 20 is driven and hot melt ink is ejected from the yellow head 21, the magenta head 22, cyan head 23, and black head 24, as appropriate, onto the intermediate transfer belt 13. In this way, a color image using hot melt inks is formed on the silicon oil applied to the intermediate transfer belt 13.
Then a sheet P is fed from a cassette (not shown) included in the ink jet printer 10 toward the sheet feed rollers 15, and further transported toward the transfer roller 16 by the sheet feed rollers 15. The time when the sheet P reaches an image forming position at the transfer roller 16 and the time when an edge of the image formed on the intermediate transfer belt 13 reaches the transfer roller 16 are controlled so as to be synchronized. The transfer roller 16 moves in a direction indicated by the arrow F3 to press the sheet P against the intermediate transfer belt 13. At that time, hot melt ink comprising the image on the intermediate transfer belt 13 is heated by the heat roller 17 and becomes soft. At the same time the softened hot melt ink is transferred onto the sheet P by the pressure of the transfer roller 16.
Then the sheet P is peeled from the intermediate transfer belt 13 by the separation pawl 18, which moves in the direction indicated by the arrow F4, and is discharged onto a tray (not shown).
However, as described above, because the silicon oil 91 applying unit 90, provided with the ink jet printer 10, must apply the silicon oil 91 to the entire area of the intermediate transfer belt 13, it can not apply the silicon oil 91 to only a specific area of the intermediate transfer belt 13.
Moreover, the thickness of the silicon oil film can not be controlled because the pressure with which the end of the felt 93 presses the intermediate transfer belt 13 is determined by the force of the coil spring 96 so that the pressure is not constant.
Furthermore, the felt 93 gets dirty ink as a result of remaining in contact with the surface of the intermediate transfer belt 13. In particular, the felt 93 gets stopped up with the ink leftover with intermediate transfer belt 13 so that silicon oil 91 can not be applied onto the intermediate transfer belt 13 evenly. Therefore, some parts of the hot melt ink do not come off completely and transfer precision declines.
As described above, the previous ink jet printer 10 is not capable of controlling the area to which silicon oil 91 is applied, the thickness of the silicon oil film, and the application of the silicon oil 91 evenly.